Yin yoga works on the yin tissues - also known as the connective tissues. Connective tissue responds best to a slow, steady load which is why we hold the poses for longer. If you gently stretch connective tissue by holding a yin pose for a long time in this way, the body will respond by making it a little longer and stronger - which is exactly what you want. Remember, the principle of exercise is to stress the tissue so the body will respond by strengthening it.